In recent years, due to reductions in the size and increases in the performance of central processing units (CPUs) which are mainly used for computers, the current density per terminal of semiconductor elements mounted in CPUs has been increasing. It is said that the current density will reach the order of around 104-105 A/cm2 in the future. As the current density increases, the heat generated by passage of current increases, thereby elevating the temperature of the terminals and increasing the thermal vibrations of atoms in the terminals. As a result, the occurrence of electromigration developed in solder joints becomes marked, eventually leading to failure of the solder joints.
Electromigration (which may hereinafter be abbreviated as EM) is a phenomenon which occurs when a current is flowing through a conductor such as a solder joint. Atoms in the solder joint which are undergoing thermal vibrations collide with electrons forming the electric current, and momentum is transferred from the electrons to the atoms, thereby increasing the momentum of the atoms. The atoms having an increased momentum migrate toward the anode side of the solder joint by going along the flow of electrons. When atoms migrate toward the anode side of the solder joint, lattice vacancies develop on the cathode side of the solder joint. These lattice vacancies accumulate to form voids. Growth of the voids eventually causes failure of the solder joint. In this manner, electromigration develops in locations where electrical conduction takes place, and it has become a problem even inside solder joints.
The environment of use of a solder joint which is envisaged in this description is an environment at the time of operating CPUs with a high current density and is referred to below as a high current density environment. Evaluation of the reliability of a solder joint in such an environment can be carried out by an electromigration test (also referred to as an EM test) in which a current with a high current density of 0.12 mA/μm2 is continuously passed through a solder joint for 2500 hours in air at 165° C.
Sn—Cu solder alloys and Sn—Ag—Cu solder alloys have been widely used as lead-free solder alloys. Sn—Cu solder alloys and Sn—Ag—Cu solder alloys easily develop electromigration because Sn, which is the main component of these alloys, has a large effective charge number. As a result, solder joints made of these alloys readily fail in a high current environment.
Patent Document 1 discloses a Sn—Ag—Cu—In solder alloy which has improved resistance to thermal fatigue and thereby suppresses the occurrence of cracks. The Sn—Ag—Cu—In solder alloy disclosed in Patent Document 1 has improved wettability due to the addition of a small amount of In. As a result, the occurrence of cracks and fracture of solder joints are suppressed.